Magnetic Resonance (MR) images that can separate tissue, bone, and air are beneficial for all applications where MR is used in combination with irradiating imaging techniques, such as Positron Emission Tomography (PET) and Single Photon Emission Computed Tomography (SPECT), and with planning for irradiating therapy techniques, such as Magnetic Resonance-Radio Therapy simulation. Unlike Hounsfield units used in CT, there is no simple relation between the MR image intensity and tissue density. For instance, using conventional MR sequences, cortical bone and air filled cavities both show no signal intensity whereas their densities are substantially different. Ultimately the ability to reliably identify additional tissue types in an MR image while the MR-acquisition time should be kept at a minimum would be beneficial.